Method for providing negative pressure to a negative pressure wound therapy bandage

ABSTRACT

A method for providing a negative pressure wherein a pump is cycled on and off to achieve a target negative pressure, which is set slightly lower than the therapeutic negative pressure. A device determines the actual pressure by averaging samples which may occur at a different rate than the pump cycle.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.61/490,118 filed May 26, 2011, the entirety of which is incorporatedherein by reference.

FIELD OF THE INVENTION

The invention relates to a method for providing negative pressure to anegative pressure wound therapy bandage.

BACKGROUND OF THE INVENTION

Negative pressure wound therapy is one method that is used to treatcertain wounds or sores on people. In general the treatment includes, abandage being placed over a wound site, and connected to a pumpingdevice. The pumping device provides suction, creating a negativepressure under the bandage at the wound site. Exudates and othermaterials are removed from the wound site, allowing the wound to healfaster than under ambient pressure.

The pumping device includes, amongst other things, a pump. Typically, auser selects (or otherwise enters) an appropriate therapeutic pressureon the pumping device that correlates to the specific negative pressuretreatment for that patient. It is important that the pump and pumpingdevice achieve the therapeutic pressure that is to be used inassociation with the negative pressure wound therapy quickly andcorrectly.

The speed at which the pump can achieve the therapeutic pressure isimportant because the speed can detrimentally effect the treatment.Moreover, the ability for the pump to recognize that the therapeuticpressure has been achieved is important to avoid applying too little ortoo much negative pressure. Thus, for a pump device, the speed and theaccuracy at which it obtains the therapeutic pressure is an importantcharacteristic.

In order to accurately and quickly achieve the therapeutic pressure,some pumps are left on continuously. In some devices this can beproblematic. For example, some pumps can quickly achieve the therapeuticpressure; however, the pump and system tends to overshoot thetherapeutic pressure value. This requires additional valves and othercomponents that allow the pumping device to release negative pressureuntil the appropriate negative pressure is obtained.

Moreover, leaving the pump on continuously can act as a power drain, andfor pumping devices that operate on batteries or other portable powersources, this can negatively impact the life time of the power source.

The present invention is directed to resolving these and other matters.

SUMMARY OF THE INVENTION

In one embodiment of the invention, the invention is directed towards amethod for providing negative pressure to a negative pressure woundtherapy bandage by providing a pumping device having a pump and settinga therapeutic negative pressure on the pumping device. The pumpingdevice determines a target negative pressure associated with thetherapeutic negative pressure and the target negative pressure is lessthan the therapeutic negative pressure (as discussed herein, “less than”means it is less negative, or a lower amount of a negative pressure, inother words, closer to zero). The pumping device is connected to anegative pressure wound therapy bandage. The target negative pressure isachieved by cycling the pump on and off to create a negative pressure inthe negative pressure wound therapy bandage, obtaining a plurality ofsamples of the negative pressure in the negative pressure wound therapybandage, each sample being obtained at a first set time interval,calculating an average of a predetermined number of consecutive samples,comparing the average to the target negative pressure, and, continuingto cycle the pump on and off until the average is greater than thetarget pressure.

The method may also include after cycling the pump on and off until theaverage of samples is greater than the target pressure after thecomplete off cycle, sampling the negative pressure in the negativepressure wound therapy bandage at a second set time interval greaterthan the first set time interval.

In another embodiment of the invention, the sampling of the negativepressure in the negative pressure wound therapy bandage at the secondset time interval begins when the pump has been off for a predeterminedamount of time.

The predetermined amount of time may be 300 milliseconds.

The second set time interval may be 1000 milliseconds and the first settime interval is 100 milliseconds.

In an embodiment, the cycling of the pump on and off to create negativepressure in the negative pressure wound therapy bandage on is performedby turning the pump on for 80 milliseconds and then turning the pump offfor 80 milliseconds.

In some embodiments of the present invention, the therapeutic negativepressure is 125 mmHg and the target negative pressure is approximately115 mmHg.

In some embodiments of the present invention, the therapeutic negativepressure is 90 mmHg and the target negative pressure is approximately 85mmHg.

In some embodiments of the present invention, the therapeutic negativepressure is 60 mmHg and the target negative pressure is approximately 54mmHg.

In some embodiments of the present invention, the therapeutic negativepressure is 40 mmHg and the target negative pressure is approximately 34mmHg.

In another embodiment of the invention, the invention is for a methodfor providing negative pressure to a negative pressure wound therapybandage by providing a pumping device having a pump and setting atherapeutic negative pressure on the pumping device. The pumping devicedetermines a target negative pressure associated with the therapeuticnegative pressure and the target negative pressure is less than thetherapeutic negative pressure. The pumping device is connected to anegative pressure wound therapy bandage. The target negative pressure isachieved by cycling the pump on and off for a first predetermined amountof time to create a negative pressure in the negative pressure woundtherapy bandage, obtaining a plurality of samples of the negativepressure in the negative pressure wound therapy bandage, each samplebeing obtained at a first set time interval, calculating an average of apredetermined number of consecutive samples, comparing the average tothe target negative pressure, and, if the average is not greater thanthe target negative pressure, repeating the steps of cycling the pump onand off, obtaining a plurality of samples, calculating an average andcomparing the average, until the average is greater than the targetnegative pressure.

In some embodiments the comparing of the average occurs at a rate fasterthan the rate of calculating the average.

In some embodiments, if the average is greater than the target negativepressure, the target negative pressure is maintained by obtaining aplurality of second samples of the negative pressure in the negativepressure wound therapy bandage at a second set time interval begingreater than the first time interval, and, comparing each second sampleto the target negative pressure, and, repeating the steps of obtainingand comparing if a second sample is greater than the target negativepressure.

In some embodiments, if a second sample is not greater than the targetnegative pressure, the step of achieving the target negative pressureby: cycling the pump on and off; obtaining a plurality of samples;calculating an average; and, comparing the average until the average isgreater than the target negative pressure, is repeated.

In the various embodiments of the present invention, the therapeuticnegative pressure may be selected from the group consisting of: 40 mmHg;60 mmHg; 90 mmHg; and, 125 mmHg.

An invention according to one or more of the disclosed embodimentsallows the pump to achieve the therapeutic negative pressure/targetnegative pressure reasonably quickly and limit exceeding the therapeuticnegative pressure/target negative pressure as much as practical.

Moreover, at least one of the embodiments also provides a reliablemethod to detect when the bandage pressure was being “topped-up” toofrequently due to a leak (i.e., often returning to cycle the pump on andoff to increase negative pressure). It is expected that the system willleak a small amount over time, for example, due to the imperfectplumbing connections in the system. A leak rate of one “top-up” perminute is normal. However, a leak rate four times that is cause for aminor leak to be declared. Such a leak rate is still within the boundsof the system to handle long term but the user should be informed sothat it is corrected and system battery life is extended. It is alsodesirable to clear the fault as soon as it appears that the leaking hasreturned to normal levels.

It is to be understood that the aspects and objects of the presentinvention described above may be combinable and that other advantagesand aspects of the present invention will become apparent upon readingthe following description of the drawings and detailed description ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully apparent from the followingdescription and appended claims, taken in conjunction with theaccompanying drawings. Understanding that the accompanying drawingsdepict only typical embodiments, and are, therefore, not to beconsidered to be limiting of the scope of the present disclosure, theembodiments will be described and explained with specificity and detailin reference to the accompanying drawings as provided below.

FIG. 1 is a front perspective view of a device used in association withone or more embodiments of the present invention.

FIG. 2 is a front side cutaway view of a device provided according toone or more embodiments of the present invention.

FIG. 3 is a flow chart of a method according to one or more embodimentsof the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

While this invention is susceptible of embodiment in many differentforms, there is shown in the drawings and will herein be described indetail one or more embodiments with the understanding that the presentdisclosure is to be considered as an exemplification of the principlesof the invention and is not intended to limit the invention to theembodiments illustrated.

Reference throughout this description to features, advantages, objectsor similar language does not imply that all of the features andadvantages that may be realized with the present invention should be orare in any single embodiment of the invention. Rather, languagereferring to the features and advantages is understood to mean that aspecific feature, advantage, or characteristic described in connectionwith an embodiment is included in at least one embodiment of the presentinvention. Thus, discussion of the features and advantages, and similarlanguage, throughout this specification may, but do not necessarily,refer to the same embodiment.

As shown in the attached FIGS. 1 and 2, the present invention isdirected towards a method used with a pumping device 10 having a pump20. The pumping device 10 generally includes a pressure sensor 22, acheck valve 24, a control circuit 26 (with microcontroller), internalplumbing 28, LEDs 30 and a power source 32.

In order to provide negative pressure to a wound, the pumping device 10is connected to a negative pressure wound therapy bandage 12, typicallywith tubing 14.

In order to achieve a target negative pressure the pumping device 10turns the pump 20 on for short amounts of time when the pressure isbelow the target pressure.

The pump device 10 typically operates as follows: a microcontrollerturns the pump 20 on via a digital output; the pump 20 starts producinga vacuum; the vacuum is converted to a voltage by a pressure sensor 22;the voltage is converted by an analog to digital converter (“the A/D”)to a standard, recognized pressure engineering unit; and, themicrocontroller compares the monitored pressure to the target pressure.

In order to achieve the target negative pressure in a short amount oftime and avoid overshooting the target negative pressure, the pump 20 iscycled. The duty cycle, the percentage of pump on time relative to pumpoff time, after examining several pumps in action, can be constant for avariety of pumps 20. How much vacuum the pump 20 can draw in a shorttime period will influence the pump on time. Moreover, bandage 14 andplumbing 28 elasticity and how quickly the pressure sensor 22 and theA/D can take a measurement will influence the pump off time.

In one embodiment, the pressure sensor 22 measurement time is 20milliseconds; while the A/D measurement time is 16 microseconds.Therefore, since a change in pressure must first be converted by thepressure sensor 22 and then the A/D, the minimum measurement time is20.016 milliseconds.

It is believed to be advantageous to average readings before they areused to mitigate occasional measurement system anomalies. The number ofsamples in the average, on the one hand, when it tends to be low, dosome anomaly mitigation but also remain the most responsive to quickchanges in the parameter they are measuring. The best anomaly mitigationappears to occur when the number of samples is high, but then theaveraging is not very responsive to quick changes in the parameter theyare measuring.

It was determined that it would be advantageous to use a sliding 5sample average. If the pressure were sampled continuously, a completeaverage would be ready in: 5×20.016=100.090 milliseconds. However, itwould not be necessary to wait for all 5 samples to be acquired before apump control decision was made on the value because just a few samplesstill mitigates against system anomalies sufficiently. However, samplingthe pressure continuously, especially since the pressure sensor 22places the largest mA load of any component (other than the pump 20) onthe system would unnecessarily shorten battery life for too little gainin pump control.

Consequently, a measurement process asynchronous to the pump on/offcycle was used wherein pressure samples are contributed to the averageevery 100 milliseconds, and if the pump on/off control logic comparedthe average every 80 milliseconds it would compare it often enough todetermine an average affected by the very last contribution that wasmade to that average. This can be achieved by: starting the processevery 100 milliseconds; waiting the 20 milliseconds pressure sensormeasurement time; then, measuring the pressure with the A/D.

It is contemplated to synchronize the completion of the A/D measurementto the pump on/off control logic exactly but software design bestpractice says that dependencies between software modules should beavoided where possible and synchronization is not necessary here—thepump on/off control logic (cycle) will see every change any contributionmakes to the average.

When the pump 20 has met its target pressure, the A/D sampling andcomparing process detailed above changes from, for example, a repeatrate of 100 milliseconds to a repeat rate of 1000 milliseconds when thepump has been off for a sufficient amount of time. This further savesbattery life at a time when the pressure is not likely to change verymuch. The period of time the pump 20 has to be off to switch to theslower repeat rate may be, for example, 300 milliseconds; however, itmay be any value that was larger than the off period in the pump on/offduty cycle (more later on the duty cycle). Moreover, it should be atleast that long to distinguish the “off” that is due to the pump 20having reached the target pressure and the “off” of the pump controlduty cycle. (The 300 milliseconds was utilized as it is a sufficientamount of time for the A/D to wait before it samples the battery voltageafter the pump is turned off. The battery is only sampled when the pump20 is turned off because the mA load that the pump places on the systemis significantly different than when it is off and battery life is moreeasily predicted when the pump is not loading the system.)

The pump control on/off duty cycle is preferably 80 milliseconds on and80 milliseconds off. It was determined that an “on” time of 40 or 80milliseconds was sufficient for constraining the overshoot for a varietyof bandages 12. The 40 milliseconds value constrained it better, yieldedend pressures closer to the target pressure, but took longer to get tothe target pressure. The “on” times longer than 80 milliseconds createdout-of-bounds overshoot for some bandages—no matter what the “off” timewas. As mentioned above, the “off” time has an effect on the measurementpart of the control/measurement algorithm. Further, plumbing and bandageelasticity, although minor, can contort the average or “steady” pressurefor a short while. It is believed that it is advantageous to let suchpressure contortions settle before measurement. Moreover, the pressuresensor 22 and A/D measurements need time also as noted above.

An “off” time of approximately 30 milliseconds would cover both but itwas convenient to use the same 80 milliseconds timer to control both theon time and the off time. Further, letting the system settle longer, forone or more multiples of the 80 milliseconds, would improve the qualityof the measurement because it would let the sliding average fill morecompletely—at the cost of taking longer to get to the end pressure.While these times have been discussed, nothing herein should beconstrued to limit the present invention to these times.

In use, a user inputs the therapeutic negative pressure into the pumpingdevice. Of course, this can be accomplished with the pumping devicehaving pre-set therapeutic negative pressures and the user merelyselects one.

In one or more embodiments of the present invention, a target negativepressure is set slightly less than the therapeutic negative pressureentered by the user into pumping device. In a working example of anembodiment of the invention, the following parameters were used with theobject to obtain the therapeutic negative pressure of 40/60/90/125 mmHgwithin 15 seconds and limit overshoot to +/−10% of the therapeuticnegative pressure. However, for the 40 mmHg therapeutic negativepressure, the tolerance is preferably +/−10 mmHg (not +/−10%).

Using a target negative pressure that is set slightly less than the40/60/90/125 therapeutic negative pressure allows for compensation ofthe system's tendency to undervalue the actual negative pressure withinthe bandage. Therefore, the target pressures contemplated to be usedwith the previously mentioned therapeutic negative pressure values are34, 54, 85 and 115 mmHg (respectively). Other target pressures can beused depending on the variety of parameters discussed above (pump,bandage, elasticity, etc.) without departing from the spirit of thepresent invention.

In a device according to one or more embodiments of the presentinvention, the pump 20 will reasonably quickly achieve the targetnegative pressure, stop, “top-up” once or twice after a few seconds andtransition to the state where it only tops-up every minute or so tocompensate for system leaking. The initial one or two top-ups are due tothe pressure completely settling.

In sum, in one or more embodiments of the present invention, pressure issampled every 100 ms; with the sampling by the pressure sensor and A/Dtaking 20.016 milliseconds. Samples go into a sliding 5 element average.The pumping device 10 asynchronously compares the average every 80milliseconds s (slightly faster than the pressure is sampled). Thepumping device 10 turns the pump 20 on for no more than one cycle (80milliseconds) if pressure is below target pressure. After waiting oneoff cycle (80 milliseconds), if the pressure is still below the targetpressure, the pump 20 is turned on again. These last two steps repeatuntil the pressure is greater than the target pressure, at which pointthe pump 20 is left off until the normal leaking in the system reducesthe pressure below the target pressure. During the period when thepressure is above the target pressure, starting when the pump 20 hasbeen continuously off for 300 milliseconds, sampling may be reduced tothe 1000 milliseconds rate (to extend battery life).

A flow chart for performing the steps of some embodiments is shown inFIG. 3. As shown therein, and as discussed above, a method according toone or more embodiments of the present invention may begin with thesetting of a therapeutic negative pressure 100. In response to same, thepumping device will determine a target negative pressure 102. Aspreviously discussed the target negative pressure is less than thetherapeutic negative pressure.

At this point, the pumping device will begin to cycle the pump on andoff 104. In addition, the pumping device will also begin sampling thepressure at the bandage 106. This sampling is preferably done at a settime. In FIG. 3, this is shown as being at “Timing A.” The pumpingdevice determines if there are at least five samples measured 108. Ifless than five samples have been collected 100, the pumping devicecontinues to sample the pressure at the bandage 106.

If, however, more than five samples have been collected 112, the pumpingdevice will calculate the average of the last five samples taken 114.The pumping device will then compare the average of the last fivesamples to the target negative pressure 116. If the average of the lastfive samples is less than the target negative pressure 118, the pumpingdevice will continue to sample the pressure at the bandage 106.

If the average of the last five samples is greater than the targetpressure 120, the pumping device will cease the pump cycling. It iscontemplated, although not required, that if the pump is in the middleof a cycle, that specific cycle be completed, before the pumping cyclingis stopped. Alternatively, the pump cycling can be immediately stoppedwhen the average is determined to be greater than the target negativepressure.

After the pump cycling has been stopped, the device begins sampling thepressure at the bandage 124. This sampling is conducted, preferably at apredetermined time interval, and as shown, occurs at “Timing B.” Aspreviously mentioned, in order to increase the life of the batteriesthat may be powering the pumping device, it is preferred that Timing Bis greater than Timing A.

Finally, the device continues to compare the sample of the pressure(taken in step 124) with the target negative pressure 126. If the mostrecent sample is greater than the target negative pressure 128, thedevice continues to sampling the pressure at the bandage 124.

If the most recent sample is less than the target pressure 130, thedevice may turn the pump on for one cycle 132, and continue to samplethe pressure under the bandage 124. Alternatively, the device can returnto cycling the pumping on and off 104 and the subsequent stepspreviously taken to achieve the target negative pressure.

In addition to achieving and maintaining negative pressure, one or moreembodiments of the present invention also include a method for detectinga leak. For example, if the target negative pressure has been reachedand at least 20 top-ups have occurred at the target negative pressureand the average of the last N (=3) top-up intervals is less than 15seconds, a leak may be declared by the pump device. The minor leakdetection may be cleared if the pump is disabled by the user or thetop-up intervals are greater than or equal to 15 seconds.

The 15 seconds time value is related to an unacceptable leak rate (4top-ups per minute) that was determined to be optimal. The pumpingdevice records the time (since unit reset) of every top-up into acircular array of N elements and checks the top-up interval at everytop-up or every 80 milliseconds, whichever occurs first. (The “or 80milliseconds” facet is more useful for clearing the fault than declaringit.)

The time in the oldest of the array elements is subtracted from thecurrent time and then divided by N to either yield the average of thelast N top-ups (or what the average would have been if the 80milliseconds timer tick that prompted the check was a top-up). Anexample of setting the fault and clearing the fault is given below:

TABLE ONE Setting the fault (assume 17 top-ups for this pressure havealready occurred): Top-Up # Time Note 18 05:42:23.000 19 05:42:33.000 2005:42:43.000 Check called due to this top-up. System time is 05:42:43.System time−oldest top-up time = 00:00:20 00:00:20/3 = 6 6 < 15, faultdeclared

The checks called due to the 80 milliseconds timer expiring are notshown in the above because they would have had no effect and theiromission makes it easier to understand how the checks due to just thetop-ups take place.

TABLE TWO Clearing the fault (assume a continuation of the above TABLEONE) Top-Up # Time Note 18 05:42:23.000 19 05:42:33.000 20 05:42:43.000Check called due to this top-up. System time is 05:42:43. Systemtime−oldest top-up time = 00:00:20 00:00:20/3 = 6 6 < 15, fault declared05:42:43.080 Check called due to 80 ms timer tick, System time is05:42:43.080 System time−oldest top-up time = 00:00:20.08000:00:20.080/3 = 6 6 still less than 15, fault stands 05:42:43.160Similar to the previous, fault stands etc No top-ups etc No top-ups05:43:08.000 Check called due to 80 ms timer tick System time is05:43:08.000 System time−oldest top-up time = 00:00:45 00:00:45/3 = 1515 is not less than 15, fault cleared.

In the example above, the fault was cleared within 25 seconds of thelast top-up, a fraction of three times the 15 seconds threshold (thethreshold used to declare the fault). This is consistent with the goalof clearing the fault quickly if it appears as though the fault has beencorrected. It could have been cleared even sooner, as fast as within 15seconds, if the system time was compared to the newest recorded top-upinstead of the oldest—but using more than one top-up time gives greaterconfidence that the fault has really been cleared.

It is to be understood that additional embodiments of the presentinvention described herein may be contemplated by one of ordinary skillin the art and that the scope of the present invention is not limited tothe embodiments disclosed. While specific embodiments of the presentinvention have been illustrated and described, numerous modificationscome to mind without significantly departing from the spirit of theinvention, and the scope of protection is only limited by the scope ofthe accompanying claims.

1. A method for providing negative pressure to a negative pressure woundtherapy bandage, comprising the steps of: providing a pumping devicehaving a pump; setting a therapeutic negative pressure on the pumpingdevice, wherein the pumping device determines a target negative pressureassociated with the therapeutic negative pressure, the target negativepressure being less than the therapeutic negative pressure; connectingthe pumping device to a negative pressure wound therapy bandage; and,achieving the target negative pressure by: cycling the pump on and offto create a negative pressure in the negative pressure wound therapybandage; obtaining a plurality of samples of the negative pressure inthe negative pressure wound therapy bandage, each sample being obtainedat a first set time interval; calculating an average of a predeterminednumber of consecutive samples; comparing the average to the targetnegative pressure; and, continuing to cycle the pump on and off untilthe average is greater than the target pressure.
 2. The method of claim1 further comprising the steps of: after the step of cycling the pump onand off until the average of samples is greater than the target pressureafter the complete off cycle, sampling the negative pressure in thenegative pressure wound therapy bandage at a second set time intervalgreater than the first set time interval.
 3. The method of claim 2wherein the step of sampling the negative pressure in the negativepressure wound therapy bandage at the second set time interval beginswhen the pump has been off for a predetermined amount of time.
 4. Themethod of claim 3 where the predetermined amount of time is 300milliseconds.
 5. The method of claim 2 wherein the second set timeinterval is 1000 milliseconds.
 6. The method of claim 5 wherein thefirst set time interval is 100 milliseconds.
 7. The method of claim 1,wherein the step of cycling the pump on and off to create negativepressure in the negative pressure wound therapy bandage on is performedby turning the pump on for 80 milliseconds and then turning the pump offfor 80 milliseconds.
 8. The method of claim 7, wherein the first timeset interval is 100 milliseconds.
 9. The method of claim 1, wherein thetherapeutic negative pressure is 125 mmHg and the target negativepressure is approximately 115 mmHg.
 10. The method of claim 1, whereinthe therapeutic negative pressure is 90 mmHg and the target negativepressure is approximately 85 mmHg.
 11. The method of claim 1, whereinthe therapeutic negative pressure is 60 mmHg and the target negativepressure is approximately 54 mmHg.
 12. The method of claim 1, whereinthe therapeutic negative pressure is 40 mmHg and the target negativepressure is approximately 34 mmHg.
 13. A method for providing negativepressure to a negative pressure wound therapy bandage, comprising thesteps of: providing a pumping device having a pump; setting atherapeutic negative pressure on the pumping device, wherein the pumpingdevice determines a target negative pressure associated with thetherapeutic negative pressure, the target negative pressure being lessthan the therapeutic negative pressure; connecting the pumping device toa negative pressure wound therapy bandage; and, achieving the targetnegative pressure by: cycling the pump on and off for a firstpredetermined amount of time to create a negative pressure in thenegative pressure wound therapy bandage; obtaining a plurality ofsamples of the negative pressure in the negative pressure wound therapybandage, each sample being obtained at a first set time interval;calculating an average of a predetermined number of consecutive samples;comparing the average to the target negative pressure; and, if theaverage is not greater than the target negative pressure, repeating thesteps of cycling the pump on and off, obtaining a plurality of samples,calculating an average and comparing the average, until the average isgreater than the target negative pressure.
 14. The method of claim 13wherein the step of comparing the average occurs at a rate faster thanthe rate of calculating the average.
 15. The method of claim 13 furthercomprising the step of: if the average is greater than the targetnegative pressure, maintaining the target negative pressure by:obtaining a plurality of second samples of the negative pressure in thenegative pressure wound therapy bandage at a second set time intervalbegin greater than the first time interval; and, comparing each secondsample to the target negative pressure; and, repeating the steps ofobtaining and comparing if a second sample is greater than the targetnegative pressure.
 16. The method of claim 15 further comprising thestep of: if a second sample is not greater than the target negativepressure, repeating the step of achieving the target negative pressureby: cycling the pump on and off; obtaining a plurality of samples;calculating an average; and, comparing the average until the average isgreater than the target negative pressure.
 17. The method of claim 13wherein the therapeutic negative pressure is selected from the groupconsisting of: 40 mmHg; 60 mmHg; 90 mmHg; and, 125 mmHg.